Design of backdoor

Design of Back Door

Aim:

To design the Back door (Tail gate) of a car from the styling surface provided using NX CAD. The ultimate aim is to design the Back Door by following the Industrial Standards.

Back Door:

A back door is a BIW component found in all Automotive car generally used for opening and closing of the so called car luggage that is usually used for storing items inside a car. In general the opening and closing of the door is co-ordinated by the Latch and Stricker position and the Gas stay position that is mounted on the body and back door of the car. These positions play a major role in ensuring smooth opening and closing of the tail gate.

Parts of a Tail Gate:

• Outer Panel
• Outer panel Lower
• Inner panel
• Hinge Reinforcements
• Latch and Stricker Reinforcement
• Wiper Motor Reinforcement
• Gas Stay Reinforcements

All these parts are assembled together to obtain the back door of a car.

mercedes benz

Types of Back Doors:

There are three types of back door based on the position of the Hinge axis. They are

• Top Mounted

• Side Mounted

• Bottom Mounted

1956 Chevrolet nomed

In our design we follow the Top Mounted Design where the Hinges are mounted on the top.

Design Considerations In Back Door Design:

The following are the Design considerations taken while designing the Back Door. They are,

• Visibility Criteria
• Force Dissipation
• Latch and Stricker Positions
• Hinge Position
• Gas Stay Position
• Material Selection
• Space Availability
• Hemming and Relief Areas

All these design considerations go hand in hand with eachother because these positions are dependant on each other and if position gets altered all the positions has to be altered for effective opening and closing of the back door. The Latch and Stricker, Hinge Position and the Gas stay Positions has to be maintained in a proper way so that they play a major role in the opening and closing as well as the rear Visibility criteria of the car.

Material Selection:

The material selection play a major role in determing the overall weight of the back door. The materials generally used are Steel and Aluminium. The Density of Steel is higher than Aluminium thus it results in adding more weight of the door. Basically the material is used based on the budget of the car, because the cost of steel very cheap than Aluminium. If aluminium is used instead of steel there is no need of many parts like Reinforcements for each mounting and the overall weight geta reduced by 6-10 kg approximately. But the draw back is Aluminium is much costlier than Steel. Hence based on the budget of the car aluminium is used. In higher end models like Benz, Audi, Porshe and BMW we can find aluminium is used instead of steel resulting in reduced weight and increased performance of the vehicle.

Another Factor to make a note is using Casted Aluminium instead of deep drawn Steel components and adding reinforcements to it is the space of the luggage commodity gets increased by a certain amount because the draft angle for Casted aluminium is much lesser (3 degree) than the deep drawn Steel inner panel (7-9 degree). Thus it reduces the space between the inner and outer panel resulting in much larger cabin space than the deep drawn steel inner panel. Thus the material selection is one of the design consideration taken. Since it reduces the space between the inner and the outer panel the rear visibility of the car is also inproved giving more space for the mirror to view.

Hence in this design the material chosen is steel since it is designed for a medium budget car and hence high cost could not be afforded to design the back door.

Force Dissipation:

The major criteria in designing the back door or any BIW part is the force dissipation during an impact or a colloision. Hence studies are being done inorder to dissipate the force on the sides and avoiding it reach the inner cabin of the car so that possible damage of imapct inside the car is reduced. Hence inorder to do that the V or Y embosses are created on the inner panel so that the force gets transmitted on to the sides instead reaching it on the cabin. This is one of the main criterias in back door design.

Thus from the above image it is clear that the forces are dissipated on the sides instead of reaching inside. This is the reason to have Y or V embosses on the panel to increase the stiffness and to dissipate forces onto the sides.

Latch and Stricker Positions:

The Latch and Stricker positions play a major role in determing the opening and closing of the back door. Hence the opening and closing of the had to smooth and inorder to ensure that the the centre point of the latch and stricker has to to be at the same point and the trajectory has to be tangent to the open close circle that is drawn. This is shown in the below image. Only if the centre points and the tangent line are perfect the opening and closing of the back door is smooth and clear. Or else the Hinge positions, Gas stay positions and Latch and Stricker Positions has to be altered to meet the change.

Hemming and Relief Areas:

Hemming and Seaming are two similar metal working processes in which sheetmetal edge is rolled onto itself.Hemming is the process in which the edge is rolled flush to itself, while a seam joins the edge of two materials.

The Hemming is done on the outer and the inner panel with an clearance of 0.2mm on the either sides. Since at the corners the hemming is difficult to perform, clerances has been given at the ends of panels so that it would for the hemming operstion is performed. Here is the image of the hemming relief at the corners.

Parts of the Back Door Assembly:

1) Outer Panel

2) Inner Panel

3) Reinforcements

Reinforcements:

Reinforcements are generally used to increase the strength of the mountings so that there is no unnecessary dents or noise occurs at the time of motion. This not only add strength to the part but also increase the overall stiffness of the part so that the part is completely stable. The Reinforcements used are

1) Latch and Stricket Reinforcement

2) Wiper Motor Reinforcement

3) Hinge Reinforcement

4) Gas Stay Reinforcements

The Following Image shows the reinforcement that are used on different areas of the inner panel. They are usually spot welded on the inner panel.

Back Door Assembly:

Thus the top down approach is followed to design the back door assembly. At first from the given styling surface the inner panel was created by Offseting the outrer Panel and then individual parts were designed accordingly by using the Inner panel Surfaces. The Reinforcement and other commodities were designed by using the inner surface as the reference thus following the top down approach. The following image show the assembly model of the Back Door.

Mastic Sealants:

The highlighted region in the image is the region where both the inner and outer panel are joined at the centre with the help of Mastic Sealants. There is a gap of 5mm for the mastic sealants to melt and gets fixed at the surface of the panels.

Conclusion:

Thus the designing of the Back Door or Tail Gate is done as per Industrial standards and Manufacturing Techniques.